Clinical Anatomy of the Ear – Flashcards

Sound enters the external auditory meatus, travels down the canal where it focuses on the tympanic membrane. The sound vibrations are amplified by the middle ear ossicles (stapes, malleus, incus). The stapes sits on the oval window of the bony labyrinth that contacts the cochlea. The vibrations are transmitted to the cochlea causing mechanosensory hair cells at a particular region of the basilar membrane to be stimulated. These hair cells are innervated by the nerve fibers from CN VIII, which transmits the signal back to the brain for the perception of sound.

The external audiory meatus form the canal from the pinna to the tympanic membrane. It consists of stratified squamous epithelium continuous with the skin. It contains hair follicles, sebaceous glands and cerumineous glands. The cerumineous glands produce earwax (cerumen) which is thought to deter bacterial growth, but overproduction can lead to hearing loss

Otitis externa is an outer ear infection that manifests as a red ear canal, swelling, and irritation of the tympanic membrane (could have small perforations in the membrane). If the perferations in the membrane are large, you can get a loss of hearing.

Perforations of the tympanic membrane result from otitis externa (outer ear infection), water sports where a build-up of water in the ear could increase pressure and cause perforation of the membrane, and loud sound. When one has a perforated tympanic membrane, they should avoid doing water sports because water would leak from the external ear to the middle ear

Contains derivatives from all three germ layers: outer layer is the thin epidermis from ectoderm, middle layer is connective tissue containing collagen and fibroblasts, and inner layer is cuboidal epithelium continuous with the lining of the middle ear (mesenchyme)

The pinna, the external auditory meatus, the tympanic membrane

Air filled space between the inner and outer ear. It communicates with the pharynx via the auditory tube. This tube is normally collapsed by opens when swallowing to equalize pressure between the middle and outer ear. It communicates with the inner ear through the round and oval windows and contains three auditory ossicles: the malleus, incus, and stapes. These ossicles serve to conduct and amplify sound waves from the tympanic membrane to the oval window. The middle ear also contains two muscles called the tensor tympani and stapedius muscle which contact the tympanic membrane and stapes to dampen vibrations from loud sounds

Otitis media (middle ear infection). The connection between the pharynx and the middle ear via the eustachian tube is more horizontal and less patent in children. This means that bacteria have an easier time traveling to the middle ear through the horizontal canal and are able to remain there because there is less drainage of this region due to the fact that the tube is not very patent.

Their eustachian tubes are less patent meaning that swallowing does not open the canal and the middle ear is unable to equalize with the outer ear leading to a build-up of pressure and discomfort

The stapedius muscle (CN VII) connects to the stapes and the tensor tympani (V3) connects to the malleus. These muscles contract in response to loud sound so as to minimize the vibration of the middle ear ossicles. These muscles do not work well when they first hear the sound and so rapid, loud noises can lead to loss of hearing as can prolonged exposure to loud noises

The malleus; the stapes

With otitis media, the tympanic membrane pouches outward.

Primary treatment: antibiotics Recurrent infection: Do a tympanostomy where tubes are placed in the tympanic membrane to provide an alternate route for drainage of the middle ear. When a child has these in place, they cannot go swimming. You can also drain the middle ear by making an incision in the tympanic membrane. This is called a Myringotomy

It is a benign tumor of the skin that starts in the outer ear canal but penetrates the tympanic membrane to invade the middle ear. These tumors are secretory and the compounds they secrete dissolve the middle ear bones. Surgical removal of the tumor is required. If you lose the middle ear ossicles and the tympanic membrane, they will insert prosthesis to replace the middle ear bones and fascia to replace the tympanic membrane

The overgrowth of the middle ear bones usually affecting the stapes. Overgrowth of the foot plate of the stapes that connects to the inner ear via the round window leads to a fixation of the bone so that it can no longer transmit sound waves. A person with this condition would be unable to hear. To treat this condition, the overgrown bones are removed and replaced with prosthesis. This is often a genetically caused condition but it has also been found to be linked to the hormonal changes that occur during pregnancy

The chorda tympani branch of the facial nerve runs through the middle ear and is in danger of being damaged. If it is severed, it can lead to a loss of taste and facial paralysis.

Otocyst (ectoderm).

Endolymph; perilymph

The endolymph is within the membranous labyrinth. It is high in potassium and calcium concentrations. In contrast, the perilymph is located between the membranous and bony labyrinths and is low in potassium and calcium concentrations. It is important that the perilymph and the endolymph remain separate in order for the driving force on K+ to be maintained (from endolymph to perilymph).

While the cause has not been identified conclusively, it is believed that patients with Meniere's disease produce too much endolymph such that there is a buildup of pressure leading to the rupturing of the membranous labyrinth (usually at the level of the scala media of the cochlea) and the mixing of the perilymph and endolymph. This abolishes the driving force for K+ to move from the endolymph to the perilymph. The mixing of the fluids causes severe vertigo and hearing loss. Treatments include dietary changes, PT, diuretics, and labryinthectomy.

Sound: cochlea Acceleration/decceleration: Utricle and saccule Equilibrium/head movements for balance maintenance: Semicircular canals

The saccule and utricle contain the maculae which consists of support cells and two types of receptor cells. The receptor cells are lined by sterocilia and a single long kinocilium. One receptor cell type is goblet shaped and surrounded by net afferent nerve fibers while the other is columnar and has both efferent and afferent nerve endings. These receptor cells are embedded in a gelatinous membrane known as the otolithic membrane. On top of this gelatinous membrane are calcium carbonate crystals called otoliths. The hair cells are also anchored to the bony labyrinth. Because the otoliths are more dense, acceleration/decceleration causes tension in the gelatinous matrix overlying the hair cells, causing bending of the stereocilia which allows for the perception of acceleration and the adjustment for balance to be maintained.

This condition is caused by an otolith that has become free from the gelatinous membrane in the saccule or utricle and is now floating in the semicircular canals. This leads to short dizzness spells after movement of the head. A series of canalith head movements is conducted to move the otolith out of the semicircular canal and back into place.

They are epithelial cells with stereocilia bundles on their apical surfaces. These stereocilia bundles are reflected in response to sound waves or movement. Deflection leads to an influx of caclium and potassium leading to depolarization of the cell and transduction of the signal via CN VIII. These bundles are extremely sensitive and respond to small deflections allowing us to hear a wide range of sounds and perceive a wide range of motion

The semicircular canals contain the crista ampullaris which is composed of receptor cells lined by a single long kinocilium and support cells. Covering these cells is a thick gelatinous copula which extend across the ampulla and contact the opposite wall. Movement of the head in any direction will move the fluid within the semicircular canal(s) thereby exerting bending forces on the cupula. This gives a sense of angular acceleration/decleration.

It consists of the basilar membrane, hair cells, and a tectorial membrane within the cochlea.

Sound waves initiate vibrates that originate at the oval window and travel up the spiral through the scala vestibuli, the scala media and then back down through the scala tympani and out the round window. The passage of these vibrations leads to movement of the tectorial membrane and the basilar membrane that are detected by the hair cells and transmitted to the brain.

The frequency and the pitch. Low frequency sounds are perceived distantly in the cochlea whereas high frequency sounds are perceived closer to the entry of the cochlea.

The organ of chorti consists of four rows of hair cells that extend along the length of the cochlear spiral. The stereocilia are embedded in a gelatinous structure called the tectorial membrane. Sound-derived pressure waves vibrate the basilar membrane where the hair cells sit and causes a deflection of the hair cells along the tectorial membrane.

It damages the hair cells in the cochlea. Once these cells are damaged, they cannot regenerate and hearing loss can result if the damage is sufficient. Profoundly deaf people have complete loss of cochlear hair cells.

Analog: take the sound and make it louder to increase the amount of the cochlea that is stimulated by the sound Digital: programmed to take into account where the cochlea is damaged versus functional. It will take the incoming sound, adjust the frequency so that it can reach a region of the cochlea that works better

When hair cells are lost, the auditory nerve is usually preserved. This is taken advantage of by cochlear implants. To do a cochlear implant, a coiled series of electrodes is introduced into the cochlea adn is connected to a sound processor and microphone. Sounds are broken down into specific frequencies and individual electrodes are stimulated based on their position along the cochlea. This leads to the stimulation of particular nerve fibers and the perception of sound. If a child is born deaf and a cochlear implant is done within the first year of life, they can grow up to hear normally.

Oval window-->scala vestibuli-->scala media-->scala tympani-->round window

Scala media